This invention relates to the measurement of oscillator spillover noise produced in a single sideband radio system and to the determination of the contribution of intermodulation distortion to noise produced in the system.
In designing and manufacturing single sideband microwave radio systems, it is found that oscillators in the frequency translation process introduce a noise component, which can be termed "oscillator spillover noise". This noise component arises because the oscillator is not perfectly stable. The noise can be observed when a pure tone is applied as a modulating input to a transmitter. The output of the transmitter, instead of having energy only at a single microwave frequency, as would be ideal, has some energy in a relatively small bandwidth about the single frequency. This spillover bandwidth is indeed relatively small; for most microwave oscillators suitable for practical single sideband systems, the majority of the noise is within 100 KHz of the single microwave frequency.
It is important to measure oscillator spillover noise in the design, manufacture and maintenance of single sideband radios, in order to obtain the best performance from the radio. To make this measurement, the amount of oscillator spillover noise must be determined in the presence of other forms of noise, including intermodulation distortion. In the past, the amount of such noise has been determined by measuring phase noise and integrating spillover effects into a quiet channel. This method, in general, is quite tedious.
The present invention provides an easier method of measuring oscillator spillover noise and permits determination of the amount of intermodulation distortion present.